Valve stiction is one of the most common causes of oscillations in industrial process control loops such oscillations can degrade the overall performance of the loop and eventually the final product quality. The detection and quantification of valve stiction in industrial process control loops is thus important From previous Studies in the literature. a sticky valve has been shown to have a distinct signature of the suction phenomena in its valve positioner data However, the position of the modulating control valves is seldom available. We consider the problem of estimating the valve position as all unknown input estimation problem. In this work, we propose a novel application of the unknown input estimator in order to estimate the valve position given the process model and the data of the process variable and controller out-put. Using the estimated valve position. we can detect and also quantify the amount of stiction. We demonstrate the efficacy of the method through simulation examples where a sticky valve is deliberately introduced in the closed loop using a two-parameter stiction model available in the literature. Application of the proposed methodology to a laboratory scale flow control loop is presented An industrial case Study is also presented in which the algorithm accurately detects and quantifies stiction in the level control loop of a power plant. (C) 2009 Elsevier Ltd All rights reserved.